We define the Maslov index of a loop tangent to the characteristic foliation
of a coisotropic submanifold as the mean Conley--Zehnder index of a path in the
group of linear symplectic transformations, incorporating the "rotation" of the
tangent space of the leaf -- this is the standard Lagrangian counterpart -- and
the holonomy of the characteristic foliation. Furthermore, we show that, with
this definition, the Maslov class rigidity extends to the class of the
so-called stable coisotropic submanifolds including Lagrangian tori and stable
hypersurfaces.